This invention relates to an organic electro-luminescence (hereinafter referred to also as EL) element, and a display, and particularly to an organic electroluminescence element excellent in luminance of emitted light and a display comprising the organic electroluminescence element.
As an emission type electronic displaying device, there is an electroluminescence device (ELD). As materials constituting the ELD, there is an inorganic electroluminescence element or an organic electroluminescence element. The inorganic electroluminescence element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic electroluminescence element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a positive hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by deactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several to several decade volts. Further, the element has a wide viewing angle and a high visuality since the element is of self light emission type, and the element is a complete solid element, and the element is noted from the viewpoint of space saving and portability.
However, in the organic EL element for practical use, an organic EL element is required which efficiently emits light with high luminance at a lower power.
In U.S. Pat. No. 3,093,796, there is disclosed an element in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound.
An element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Japanese Patent O.P.I. Publication No. 63-264692), and an element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190). The organic EL element comprising a host compound doped with such a fluorescent compound with high fluorescence quantum yield provides high luminance as compared with conventional organic EL elements.
However, light from the fluorescent compound in a small amount with which the host compound is doped is emitted through excited singlet state. When light emitted through excited singlet state is used, the upper limit of the external quantum efficiency (next) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%. Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (M. A. Baldo et al., Nature, 395, 17, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made. (M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000), and U.S. Pat. No. 6,097,147 etc.)
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
It has been found that it is necessary that when a phosphorescent compound is used as a dopant, the maximum emission wavelength of light which a host compound emits, be in the region shorter than the maximum emission wavelength of light which the phosphorescent compound emits, and in addition, there exist other requisitions to be satisfied.
Several proposals with respect to the phosphorescent material were made in xe2x80x9cThe 10th International Workshop On Inorganic and Organic Electroluminescence (EL ""00, Hamamatsu)xe2x80x9d. For example, Ikai et al. use, a hole transporting compound as a dopant of a phosphorescent material, M. E. Tompson et al. use, as a host compound of a phosphorescent material, various kinds of electron transporting compounds, which are doped with a new iridium complex, and Tsutsui et al. obtain high light emission efficiency due to introduction of a hole blocking layer.
The host compounds of phosphorescent compounds are disclosed in for example, C. Adachi et al., xe2x80x9cAppl. Phys. Lett., 77, pp. 904 (2000)xe2x80x9d, but an approach from a new aspect with respect to characteristics required in the host compounds is necessary to obtain an organic electroluminescence element emitting light with high luminance. However, any technique described above does not provide an organic EL element providing both high emission luminance and long emission lifetime.
The present invention has been made in order to obtain an organic electroluminescence element providing high emission luminance and long emission lifetime. Accordingly, an object of the present invention is to provide an organic electro-luminescence element emitting light with high emission luminance and long emission lifetime, and a display emitting light with high emission luminance and long emission lifetime, which employs the organic electroluminescence element.